Good news does not always flow freely in India. Too many children still go hungry. Violence against women endures. Inflation is soaring, and gay sex was just criminalized, again.

But today India got a boost: Jan. 13 marks the country’s third year of being free of polio, the highly infectious disease that attacks the nervous system of children in particular and can paralyze within hours. The last child to be crippled by polio in India was a 2-year-old girl in West Bengal, whose case was confirmed on Jan. 13, 2011. The fact that none have been found since is a stunning turnaround from 2009, when India hosted nearly half the world’s cases. That polio has been wiped from this vast, crowded country is arguably one of the greatest achievements in modern public health — and a stirring reminder that sheer determination can, in fact, change lives.

People used to say that ridding India of polio simply couldn’t be done. The virus has used the subcontinent as an incubator for centuries, and some experts argued that the slow process of vaccinating every child could never outpace the rapid transmission of the disease. Happily, they were wrong. Teaming up with groups like Rotary International, the Bill & Melinda Gates Foundation and the World Health Organization (WHO), the Indian government launched yearly national vaccination drives carried out by millions of volunteers and, eventually, backed up by sophisticated disease-surveillance and population-monitoring systems. In 2002, there were 1,600 polio cases in India. By 2009, there were 741. Today, there are none.

That accomplishment has changed the whole tenor of the global fight against the disease. In May 2012, not long after India had been declared polio-free for a year, the World Health Assembly, the decisionmaking body of WHO, declared global polio eradication to be a public-health priority. “If it can be done in India, then there is nowhere in the world that can use the argument that it is impossible,” says Dr. Hamid Jafari, director of polio operations and research for WHO. In India, Jafari helped oversee the distribution of nearly 1 billion polio-vaccine doses every year between 2008 and 2011. “When a government puts its full commitment to do it, and a financial commitment to match, then it is doable,” he says.

But infectious disease does not leave much time for resting on laurels. The virus is still endemic in three countries — Nigeria, Afghanistan and Pakistan — and as long as it is, it can still go anywhere, including back to India. Case in point: last year, new polio cases were confirmed in several countries that were supposed to be free of poliovirus — including Cameroon, Syria, Kenya, Ethiopia and Somalia — as a result of a spread from endemic countries.

“We cannot be complacent,” says Dr. Naveen Thacker, a member of the expert-advisory group to the Indian government for polio eradication. For years, India exported polio to the world. Today, Thacker says, “those channels are still open, but now we are on the receiving end.”

That one of the endemic countries is right next door isn’t much comfort. In Pakistan, the Muslim militants’ ban on the polio vaccine since June 2012 in swaths of the restive northwest is currently allowing the virus to be freely transmitted among some 300,000 unvaccinated children. Workers trying to distribute the vaccine have been attacked and killed in parts of the country.

“If we got access to 100% of the children, we could stop the virus transmission,” says Dr. Shamsher Khan, UNICEF’s coordinator for polio for high-risk populations in Pakistan. He says most of the 85 cases recorded there last year were from these inaccessible zones, but seven of the new cases were found in Punjab, a province where the virus had previously been controlled. “You can see the spillover,” Khan says.

That polio could slip over the border to India isn’t a huge risk, given that old tensions mean movement between the neighbors remains limited. But that doesn’t mean India won’t have to stay on high alert. From Feb. 14, proof of polio vaccination will be required for all travelers entering India from Pakistan and Afghanistan, among other countries. Authorities will also keep monitoring India’s own population and make sure vaccination remains routine. One day, says Thacker, those efforts also may be slowly scaled back, but for now, there is still work to be done. “At the end of a war, you can withdraw your army,” Thacker says. Until then, the battle continues.